Here, \(P_T\)  is the transmit power, \(\sigma ^2\)  is the receiver noise power, and \(d_{SR}\)  and \(d_{RD}\)  respectively stand for source-RIS/RIS-destination distances, where the source-destination link is assumed to be unavailable. Two important conclusions can be drawn from (1). First, the received SNR increases with a factor of \(N^2\) , which has been known since the early RIS studies [1]}, thanks to the coherent combining of forward and backward links of the RISs with intelligent phase adjustment. However, the received SNR decays with a factor of \(d_{SR}^{2} d_{RD}^{2}\) , which considerably hurts the RIS-assisted systems in longer distancesFor clarity of presentation, we dropped higher-order \(\pi \)  and wavelength \((\lambda \) ) terms in (1), while they cannot be ignored in a complete link-budget analysis.. This is the reason why several research teams in our community, including ours, have been trying to find optimum RIS locations to boost their effect on the received signal. One conclusion of this work is that, if there exists a strong direct link, possibly with a LOS component that decays only with \(d_{SD}^2\) , where \(d_{SD}\)  is the source-destination distance, it becomes very challenging to find sweet spots to operate the RIS since its effect on the received SNR dwindles remarkably. In other words, considering the fact that the direct link SNR is given by \(\gamma _{\text{d}} \propto P_T /(d_{SD}^2 \sigma ^2)\) , it would be challenging to compete with it if the RIS size is not extremely large and/or \(d_{SR}\)  and \(d_{RD}\)  are more than several tens of meters. Consequently, most recent practical campaigns generally position RISs either close to the transmitter or the receiver, to compensate this double path loss effect [2]}. Unfortunately, we cannot foresee an easy solution to this low end-to-end path gain problem due to the underlying physics of RISs unless we consider active ones.
